Arthritis (from Greek arthro-, joint + -itis, inflammation; plural: arthritides) is a group of conditions where there is damage caused to the joints of the body. Arthritis is the leading cause of disability in people over the age of 55.

There are many different forms of arthritis, each of which has a different cause. Rheumatoid arthritis and psoriatic arthritis are autoimmune diseases in which the body is attacking itself. Septic arthritis is caused by joint infection. Gouty arthritis is caused by deposition of uric acid crystals in the joint that results in subsequent inflammation. Additionally, there is a less common form of gout that is caused by the formation of rhomboidal shaped crystals of calcium pyrophosphate. This form of gout is known as pseudogout. The most common form of arthritis, osteoarthritis is also known as degenerative joint disease and occurs following trauma to the joint, following an infection of the joint or simply as a result of aging. There is emerging evidence that abnormal anatomy may contribute to early development of osteoarthritis.

All arthritides feature pain. Patterns of pain differ among the arthritides and the location. Osteoarthritis is classically worse at night or following rest. Rheumatoid arthritis is generally worse in the morning; in the early stages, patients often do not have symptoms following their morning shower. In elderly people and children, pain may not be the main feature, and the patient simply moves less (elderly) or refuses to use the affected limb (children).

Elements of the history of the pain (onset, number of joints and which involved, duration, aggravating and relieving factors) all guide diagnosis. Physical examination typically confirms diagnosis. Radiographs are often used to follow progression or assess severity in a more quantitative manner.

Blood tests and X-rays of the affected joints often are performed to make the diagnosis.

Screening blood tests may be indicated if certain arthritides are suspected. This may include: rheumatoid factor, antinuclear factor (ANF), extractable nuclear antigen and specific antibodies.

Treatment options vary depending on the type of arthritis and include physical and occupational therapy, lifestyle changes (including exercise and weight control), and medications (symptomatic or targeted at the disease process causing the arthritis). Arthroplasty (joint replacement surgery) may be required in eroding forms of arthritis.